The present invention relates to a process for recovering the exhaust heat taking aim at the improvement of thermal efficiency, when the moderate heat energy is converted into work. In more detailed, the present invention relates to the process for recovering the exhaust heat using the working fluid of the toluene-water system or the benzene-water system in the vapor cycle, when the heat energy of 400.degree.-750.degree. C. is converted into work.
For the purpose of improvement of thermal efficiency in the power generation from fossil fuels, the combined cycle being combined the gas turbine with the steam cycle is expected as the most realizable and expected process. However, at present, the thermal efficiency of the steam cycle for recovering the exhaust heat of about 500.degree. C. coming out from the gas turbine is low. In the present technical level of the gas turbine, the thermal efficiency of said combined cycle is about 40% slightly above the thermal efficiency of conventional boiler turbine unit. Therefore, it has been the problems of the research and development to improve the thermal efficiency of the gas turbine by operating the gas turbine at the high temperature, and to improve the thermal efficiency of the steam cycle due to increasing the temperature of the exhaust heat coming out from the gas turbine.
The present invention aims at the improvement of recovering the exhaust heat independent of the improvement of performance of the gas turbine. As the result of studying the various sorts of substances other than steam suitable for the working fluid of the vapor cycle, the present invention has been accomplished by finding the fact that the mixture of toluene (or benzene) and water is useful to use for the working fluid.
When the Rankine cycle is considered to be the standard cycle of the vapor cycle, the most simple method of estimating the thermal efficiency of cycle is shown as follows:
If the inlet temperautre of turbine is Tc.degree.K.; the pressure is Pc; and the condensation temperature is Ta.degree.K., and if the vapor pressure of the saturated vapor at Ta.degree.K. is Pa (the back pressure of turbine); the heat of evaporation per one mole of the working fluid at Ta.degree.K. is La; the molecular heat of vapor is the constant value Cp, and also if the vapor submits to the rule of the ideal gas, the theoritical thermal efficiency .eta.c of the cycle is roughly estimated as follows: ##EQU1## wherein .theta.=Tc/Ta, .lambda.=La/Ta.Cp, .psi.=Pc/Pa, .alpha.=R/Cp (R: gas constant), .theta., .lambda., .psi., .alpha. are non-dimentional parameters respectively.
The outlet temperature of vapor coming from turbine Td.degree.K. is shown in the following formual. EQU Td=Tc..multidot..psi..sup.-.alpha. ( 2)
From said formula (1), the tendency of the parameter value suitable for increase the value of .eta.c is known.
In this case, in order to avoid the condensation of vapor in the low pressure stage of the turbine, it is necessary to be Td.gtoreq.Ta, whereby the value of Pc is limited by the condition of .psi..sup..alpha. .gtoreq..theta..
And, considering that the preferable condition of the working fluid is the condition in which entropy of the saturated vapor is not too much change within the operating range of the working fluid according to the temperature change, the value of .lambda. is desirable to be near to 1.
The thermal stability, the cost, the poisonous character and the corrosive property against the metal material are the fundamental terms of the working fluid. Benzene and toluene are selected for the working fluid suitable to said fundamental terms. The thermal efficiency .eta.c of the cycle is roughly calculated according to said formula. The result is shown in the table 1 in comparison with the working fluid of water.
From the values of .lambda. and .alpha. in the table 1, it has been understood that the specific characters of benzene and toluene as the working fluid are completely adverse against those of water. In the working fluids of benzene and toluene, when the value Pc is more than 100 kg/cm.sup.2 which is shown in the table 1, the value .eta.c can be increased according to the account. When the value Pc is more than 100 kg/cm.sup.2, the thermal efficiency does not substantially increase, since the power of pump for feeding the working fluid to the boiler increases. On the assumption that the value La of the working fluid is the same extent to that of benzene or toluene, it is preferable to select the working fluid having the intermediate value of Cp between that of toluene (or benzene) and that of water. As the pure substance having said intermediate value of Cp cannot be found and rather does not present, the mixture of toluene (or benzene) and water has been adopted to the working fluid of the present invention.
TABLE 1 __________________________________________________________________________ Rough estimate of the thermal efficiency .eta.c of cycle Mixture* of benzene Mixture* of toluene Benzene Toluene Water and water and water __________________________________________________________________________ Pa 35.degree. C. kg/cm.sup.2 0.201 0.0635 0.0492 0.258 0.121 La 35.degree. C. cal/mol 7919 8924 9407 9163 9665 Cp** cal/mol .degree.C. 37.74 46.58 8.639 23.19 27.61 .lambda. = La/Ta.Cp .681 .854 3.865 1.282 1.136 .alpha. = R/Cp .0526 .0426 .230 .0856 .0719 1 - .psi..sup.-.alpha. *** .279 .269 (.820)**** .400 .384 Inlet temperature 380.degree. C. 32.8 32.8 (34.9)***** 35.3 36.0 .eta.c % 400.degree. C. 32.6 32.6 (35.5) 35.4 36.1 420.degree. C. 32.5 32.4 (36.1) 35.5 36.1 __________________________________________________________________________ *1 mol:1 mol mixture **Molecular heat at 600.degree. K. ***Inlet pressure of the turbine is assumed to 100 ****In this condition, the condensation of water occurs at the low pressure side of the turbine *****The calculation value according to the formula (1)
Although the study has been carried out according to the rough calculation formula as shown above, the real vapor at high pressure is deviant from the ideal gas, and the molecular heat is also the function of temperature. Exactly, it is necessary to calculate the values of enthalpy and entropy in each condition respectively.